1. Field of the Invention
This invention relates to a disposable syringe, and more particularly to a syringe which can be used only once, to prevent its re-use particularly by drug addicts.
2. Description of the Related Art
The problem of constructing syringes which can be used only once is very urgent, as is demonstrated by the large member of patents offering different solutions to this problem.
British patent GB-A-2015883 and PCT patent WO 88/10127 describe syringes comprising a plunger fixed only weakly to its shaft and movable within the syringe cylinder, which comprises appendices projecting inwards from the cylinder. These projecting appendices are positioned close to the bottom of the cylinder where the syringe needle is mounted. In this manner, when the plunger is pushed to the bottom of the cylinder the projecting appendices engage the plunger and cause it to separate from the shaft when the shaft is pulled backwards away from the bottom of the cylinder. The drawback of this type of syringe is that it is not possible in practice to make the appendices projecting into the syringe cylinder of sufficient size to ensure reliable engagement of the plunger.
European patent EP-A-00229017 describes a syringe comprising a plunger with a shaped head engaged by a plurality of flexible deformable fingers projecting from the adjacent end of the shaft. When the syringe is used for the first time, on withdrawing the shaft it pulls the plunger back to draw the liquid to be injected into the syringe. Then when the shaft is pushed down the shaped head of the plunger deforms or breaks the flexible fingers engaged with it, after which the fingers can no longer grip the plunger to pull it away from the bottom of the syringe and allow new liquid to be drawn in.
The drawback of this syringe is that its structure is very complicated and of dubious operation because it is unclear how the plunger can be initially pushed to the bottom of the syringe cylinder without breaking the flexible fingers of the shaft, before the syringe is used for the first time to draw in the liquid.
U.S. Pat. Nos. 3,667,657, 3,951,146, and 3,998,224 comprise cutting blades or points which cut or perforate the plunger when this is pressed to the bottom of the cylinder of the respective syringe.
Patent GB-A-2205750 describes a syringe with a plunger transversed by holes below which a flexible membrane is provided to close the holes when pressing the syringe shaft, but which rises from the holes when the shaft is withdrawn from the cylinder, so preventing liquid being drawn into the syringe.
The plunger holes and the relative membrane act as automatic unidirectional valves.
U.S. Pat. Nos. 3,934,586 and 4,367,738 and patent GB-A-2117249 describe syringes in which the cylinders and plungers are shaped with projecting retention teeth and respective seats shaped to allow the shaft to move only in the direction for expelling the liquid, which has to be present in the syringe before it is used, and to prevent the shaft moving in the direction away from the syringe bottom.